In general, CMOS image sensors tend to have a limited dynamic range due to well capacity limitations. Various approaches have been attempted to improve dynamic range using such sensors, such as, for example, multiple exposure, lateral overflow, logarithmic pixel, in-pixel delta-sigma ADC, pixel with multiple-size-photodiodes, pixel arrays with different neutral density filters, dual conversion gain, etc. The multiple exposure approach is one common technique for high dynamic range (HDR) imaging.
In lateral overflow HDR schemes, pixel integration time is divided into two or more segments. In each segment, the effective pixel well capacity is varied. At the end of integration, the total accumulated charge is readout. However, this approach normally has fixed pattern noise at each knee point (the time at which an intermediary reset voltage is applied) on the signal response curve. Lateral overflow approaches also tend to be flexible regarding dynamic range extension.
Delta-sigma ADC (Analog to Digital Converter) is a method for encoding analog signals into digital signals or for encoding high-resolution signals to lower-resolution signals. In the in-pixel delta-sigma ADC scheme, each pixel's integration time is individually controlled. This approach in theory can provide the best HDR scene capture and is the most flexible to achieve the highest possible dynamic range. However, the pixel size is large, complex and not appealing for most consumer applications.
In multiple-diode-pixel systems, each pixel has multiple photodiodes. The effective sensitivity of two photodiodes can thus be made different by design. Therefore, the HDR scene multiple-exposure can be achieved simultaneously. However, the enhanced dynamic rage is fixed in design and it is not flexible.